Time-division multiplexed pilot signal for integrated mobile broadcasts

ABSTRACT

A pilot channel signal for time-division multiplexing with one or more traffic channel signals in a broadcast/multi-cast signal and for code-division multiplexing with a continuously transmitted pilot channel signal is described. In an exemplary method for transmitting a broadcast/multicast signal, a pilot symbol sequence is obtained for each slot of one or more frames of the broadcast/multicast signal, so that the pilot symbol sequence varies for each slot of a given frame. The pilot symbol sequence for each slot is spread with a channelization code, and the spread pilot symbol sequence for each slot is scrambled, using a scrambling code, to form a first pilot channel signal. The first pilot channel signal is transmitted so that it is time-division multiplexed with one or more traffic channel signals transmitted during each slot and code-division multiplexed with a second pilot channel signal transmitted during all slots of the one or more frames.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/116,455, filed Nov. 20, 2008, the disclosure of which is incorporatedherein by reference, and is a Continuation of U.S. Regular applicationSer. No. 12/572,423 filed Oct. 2, 2009, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to wireless communicationssystems, and more particularly relates to methods and apparatus fortransmitting pilot information in a broadcast/multicastsingle-frequency-network signal.

BACKGROUND

The 3^(rd)-Generation Partnership Project (3GPP) has recently developedspecifications, applicable to Release-7 Universal Terrestrial RadioAccess (UTRA) systems, for delivering so-called Multimedia BroadcastMulticast Services (MBMS) using a single-frequency network (SFN). MBMSover SFN (MBSFN) provides significantly higher spectral efficiencycompared to the MBMS approach in earlier systems (e.g., Release 6systems), and is primarily intended for broadcasting mobile televisionservices that demand high bit-rates on carriers dedicated to MBMS. SinceMBMS services are broadcast only, MBSFN is inherently suited fortransmissions in unpaired frequency bands.

With SFN transmissions, multiple base stations transmit the samewaveform at the same time. A mobile terminal can receive signals fromtwo or more of these base stations and treat the received signal as ifit was transmitted by a single base station serving a large cell. ForUTRA systems, SFN transmission implies that a cluster of timesynchronized base stations (Node B's, in 3GPP terminology), transmit thesame data, using the same channelization and scrambling codes.

Mobile terminals developed for use in Wideband Code-Division MultipleAccess (W-CDMA) systems generally use a continuously transmitted,code-multiplexed pilot signal (known as the common pilot channel, orCPICH, in 3GPP specifications) for channel estimation. Although acode-multiplexed pilot channel performs well in unicast radioenvironments, the MBSFN channel has a much larger delay spread and thusa larger number of paths to estimate in the radio receiver. Accuratechannel estimation requires long averaging over many slots. This isdifficult to realize in situations where a small duty cycle is used,i.e., where a mobile terminal's receiver is only operational duringintervals that are as short as possible. Thus, improved pilot signalsare needed.

SUMMARY

In various embodiments of the present invention, pilot sequences aregenerated based on slot-specific sequences at the symbol level, i.e.,before spreading. In several embodiments of the present invention, theseslot-specific, symbol-level sequences are then spread using anOrthogonal Variable Spreading Factor (OVSF) code. The spread pilotsequence may then be scrambled, in various embodiments of the invention,using a scrambling code (e.g., a “long code”). In several embodiments ofthe present invention, the pilot sequences are based on maximum-lengthsequences (m-sequences), which are well known sequences that have goodperiodic auto-correlation properties. In some embodiments, asymbol-level sequence is generated by pre-scrambling an input sequencethat comprises at least one instance of an m-sequence with the conjugatevalues for the scrambling code segment corresponding to the currentslot. In this way, the values of the initial sequence appear in thescrambled sequence.

In an exemplary method, a pilot symbol sequence is obtained for eachslot of one or more frames of a broadcast/multicast signal, so that thepilot symbol sequence varies for each slot of a given frame. The pilotsymbol sequence for each slot is spread with a channelization code, andthe spread pilot symbol sequence for each slot is scrambled, using ascrambling code, to form a first pilot channel signal. The first pilotchannel signal is transmitted so that the first pilot channel signal istime-division multiplexed with one or more traffic channel signalstransmitted during each slot and code-division multiplexed with a secondpilot channel signal transmitted during all slots of the one or moreframes.

In some embodiments, obtaining the pilot symbol sequence for each slotcomprises generating the pilot symbol sequence for each slot as afunction of a portion of the scrambling code corresponding to the slot.In some of these embodiments, generating the pilot symbol sequence foreach slot comprises pre-scrambling a pre-determined symbol sequence bythe conjugates of a series of values from the scrambling code, so thatthe corresponding values of the pre-determined symbol sequence appear inthe first pilot channel signal, after spreading and scrambling. In someembodiments, the pre-determined symbol sequence comprises at least oneinstance of a maximum-length sequence; various of these embodiments maycomprise, for example, two concatenated instances of a length-63maximum-length sequence, extended with two symbols to form a 128-symbolsequence, or a length-127 maximum-length sequence, extended with onesymbol to form a 128-symbol sequence.

In some embodiments, spreading the pilot symbol sequence for each slotcomprises spreading the pilot symbol sequence for each slot with anorthogonal variable spreading factor (OVSF) code, wherein the OVSF codeis selected so that the first pilot channel signal is orthogonal to thesecond pilot channel signal. In some of these embodiments, an OVSF codehaving a spreading factor of two may be used.

Processing circuits configured to carry out one or more of thetechniques summarized above are also described herein. The presentinvention may, of course, be carried out in other ways than thosespecifically set forth herein without departing from essentialcharacteristics of the invention. Upon reading the following descriptionand viewing the attached drawings, the skilled practitioner willrecognize that the described embodiments are illustrative and notrestrictive, and that all changes coming within the meaning andequivalency range of the appended claims are intended to be embracedtherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary wireless communication system utilizingmulti-carrier transmission.

FIG. 2 illustrates a channel structure for an exemplary multimediabroadcast/multicast system.

FIG. 3 illustrates an exemplary shift register for generating amaximal-length sequence.

FIG. 4 illustrates the periodic autocorrelation properties of anm-sequence.

FIG. 5 illustrates the effects of aperiodic autocorrelation in amultipath channel.

FIG. 6 illustrates the use of a repeated sequence to provide periodicautocorrelation properties in a multipath channel.

FIG. 7 illustrates elements of a processing circuit in a transmitteraccording to some embodiments of the invention.

FIG. 8 illustrates another exemplary shift register configured togenerate a maximal-length sequence.

FIG. 9 is a process flow diagram illustrating a method for transmittinga broadcast/multicast signal in a mobile communications network.

FIG. 10 is a process flow diagram illustrating an exemplary method forobtaining a pilot symbol sequence according to some embodiments of theinvention.

DETAILED DESCRIPTION

Various aspects of the present invention are described below in thecontext of specifications and standards, currently under development bythe 3^(rd)-Generation Partnership Project, for the delivery ofMultimedia Broadcast Multicast Services (MBMS) using a single-frequencynetwork (SFN), and more particularly in the context of the so-calledIntegrated Mobile Broadcast (IMB) solutions for MBMS over SFN (MBSFN)currently under discussion among 3GPP participants. Of course, thoseskilled in the art will appreciate that the techniques described hereinare not limited to application in these particular systems, and may beapplied to other wireless systems, whether already developed or yet tobe planned.

As discussed above, MBSFN has recently been specified in 3GPP forRelease 7 UTRA systems. The MBSFN feature provides significantly higherspectral efficiency compared to Release 6 MBMS, and is primarilyintended for broadcasting high bit-rate mobile television services ondedicated MBMS carriers. Broadcasting in an SFN network is illustratedin FIG. 1, which illustrates a portion of a mobile communicationsnetwork 100 that includes a mobile terminal 110 receiving transmissionsfrom two base stations 120. When cell-specific scrambling is used byeach base station 120, then transmissions from the base station on theright side would appear as inter-cell interference to mobile terminalsdemodulating and decoding transmissions from the base station 120 on theleft side, and vice-versa. In a single-frequency network transmission,on the other hand, the same scrambling and channelization codes are usedby both base stations 120. Thus, the signal transmitted from an adjacentbase station 120 becomes visible as additional multipath signals, whichcan be taken into account by the mobile terminal 110 as additionalcomponents of the desired signal.

MBSFN enhances the physical layer specifications for MBMS in 3GPP'sRelease 6 by adding support for SFN operations for MBMSpoint-to-multipoint transmissions on a dedicated MBMS carrier. The newspecifications also support higher service-bit-rates, and more efficienttime-division multiplexing of services for reducing terminal batteryconsumption by allowing discontinuous reception (DRX) of services. MBSFNuses the same types of channels that are used for Release 6 MBMSpoint-to-multipoint transmissions.

To provide smooth integration of the MBSFN feature to any existing UTRAsystem, MBSFN has been specified for downlink physical layer channelstructures for both frequency-division duplexing (FDD) and time-divisionduplexing (TDD) modes. In particular, standards have been developed foreach of UTRA's three variants, namely: MBSFN based on W-CDMA (FDD);MBSFN based on Time-Division Synchronous Code-Division Multiple Access,known as TD-SCDMA (TDD); and MBSFN based on Time-Division Code-DivisionMultiple Access, known as TD-CDMA (TDD).

The FDD-related version of MBSFN uses the WCDMA common physical layerchannels for downlink transmission of data, and no paired uplinktransmissions occur. For the TDD-related versions of MBSFN, all slotsare used for downlink transmissions when networks are optimized forbroadcast. Hence, no duplexing occurs in MBSFN in either case. Thus, thedifferences between the various versions of MBSFN are primarily limitedto the downlink physical layer slot formats, to the way mobiletelevision services are time-multiplexed, and to the particular chiprates used in the case of the TD-SCDMA and 7.68 mega-chip-per-second(Mcps) TD-CDMA. (The chip rate for the third TDD option, 3.84 McpsTD-CDMA, is the same as used in FDD.)

When multimedia services data is broadcast in all downlink slots, themeanings of TDD and FDD become obsolete, in the sense that no duplexingbetween transmission directions is occurring. Thus, as noted above, thedifferences in MBSFN operation in networks nominally designated as TDDnetworks and FDD networks is basically limited to details of theconstruction of the common downlink physical channels. Thus, animportant work item in 3GPP involves the specification of a W-CDMA-basedMBSFN approach, as a fourth TDD option in which all slots are dedicatedfor broadcast. This fourth TDD option has been referred to at variousstages as MBSFN Downlink Optimized Broadcast (DOB) or as MBSFNIntegrated Mobile Broadcast (IMB). MBSFN IMB is intended to be fullycompatible with relevant radio-frequency requirements for TDD operation.

Those skilled in the art will appreciate that although the terms IMB andIntegrated Mobile Broadcast are repeatedly used herein, these are simplyterms referring to a particular W-CDMA MBSFN solution. The inventivetechniques disclosed herein are by no means limited to systems oroperating modes referenced by those particular names, but can be appliedin a variety of contexts, under various names, such as DOB, HS-B (HighSpeed Broadcast), etc.

Existing IMB proposals for WCDMA use a code-multiplexed pilot signal(CPICH) that is continuously transmitted for use by mobile terminals inchannel estimation. Although a code-multiplexed CPICH performs well inunicast radio environments, the MBSFN channel has a much larger delayspread and thus a larger number of paths to estimate in the radioreceiver. This can be seen in FIG. 1, where multipath profiles 130-A and130-B, corresponding to the left- and right-hand base stations 120,respectively, appear to mobile terminal 110 as a single multi-pathprofile 140 having a larger delay spread than either of the constituentmulti-path profiles 130. Those skilled in the art will appreciate thataccurate channel estimation requires long averaging over many slots,which is difficult to realize in case of a short duty cycle (where theUE receiver parts are only operational during as short as possibledurations).

To improve channel estimation in MBFSN systems, participants in 3GPPspecification activities have agreed that a time-division-multiplexed(TDM) pilot sequence of 256 chips should be provided at the end of eachtransmission slot, in addition to the continuous CPICH. This TDM pilotsequence can be used to by receiving mobile terminals to improve channelestimation quality. An illustration of the resulting proposed channelformat is shown in FIG. 2, where each channel in the vertical dimensionis separated from the others by code-division multiplexing. As seen inFIG. 2, the proposed broadcast channel format may include up to fifteencode-multiplexed MBMS traffic channels (MTCHs), each transmitted with aspreading factor of sixteen (SF16). The broadcast signal furtherincludes a synchronization channel (SCH) and a primary common pilotchannel (P-CPICH), both of which are the same as previously specifiedW-CDMA channels. (Details for the SCH and CPICH formats may be found,e.g., in 3GPP TS 25.221, v8.5.0, “3^(rd) Generation Partnership Project;Technical Specification Group Radio Access Network; Physical channelsand mapping of transport channels onto physical channels (TDD) (Release8)”, May 2009.) Other channels with spreading factors of 256 (SF256) mayalso be code-multiplexed with the MTCHs, the SCH, and the P-CPICH.

Those skilled in the art will appreciate that each of the trafficchannels illustrated in FIG. 2 are transmitted during less than theentire duration of each slot. In fact, only the P-CPICH is transmittedcontinuously. Leaving a small gap at the end of each slot allows anadditional pilot signal to be time-division multiplexed (TDM) with theMTCHs and the other traffic channels. Accordingly, the TDM pilot is notimpaired by inter-cell interference from the traffic channels.

To help improve the quality of channel estimation, it is generallydesired that the TDM pilot sequences have good auto-correlationproperties. Further, it has also been agreed within 3GPP that TDM pilotsequences need to maintain orthogonality with the CPICH. The focus ofthe following discussion is techniques for forming and transmitting TDMpilot signals based on pilot sequences that are generally optimized toenable accurate channel estimation under the constraint that thetransmitted TDM pilot is orthogonal to the CPICH.

Several of the pilot sequences described below are generated based onslot-specific sequences at the symbol level, i.e., before spreading. Inseveral embodiments of the present invention, these slot-specific,symbol-level sequences are then spread using an Orthogonal VariableSpreading Factor (OVSF) code. The use of such a code, which may in someembodiments be a spreading factor 2 OVSF code of [1, −1], ensures thatthe spread pilot sequence maintains orthogonality with respect to theCPICH spread by the all 1's OVSF code. Of course, the use of spreadingcodes with other spreading factors, such as an OVSF code with aspreading factor of 16, is also possible.

Generally speaking, the spread pilot sequence may then be scrambled, invarious embodiments of the invention, using a scrambling code (e.g., a“long code”). The resulting TDM pilot sequence, which appears only atthe end of each slot as pictured in FIG. 2. For example, if a single SF2sequence is used, it has good autocorrelation properties for the evenlags, and has pseudo-random autocorrelation properties for the odd lags.

In several embodiments of the present invention, TDM pilot sequences arebased on maximum-length sequences (m-sequences), which are well knownsequences that have good periodic auto-correlation properties. Them-sequences can have length 2^(L)−1, where L is an integer numbergreater than 1. The m-sequences can be generated using a simpleshift-register architecture, such as the simple shift register designillustrated in FIG. 3. Shift register 300 includes a series of delayelements 310 configured with feedback from the last and second-to-lastdelay elements, through modulo-2 adder 320, to the input of the shiftregister. (Of course, alternative shift-register architectures are alsopossible, including those configured according to the Galoisconfiguration, rather than the Fibonacci configuration of FIG. 3.) Thosefamiliar with maximum-length sequences will appreciate that theconnections to the modulo-2 adder can be determined by a primitivepolynomial of degree L. Thus, for example, to generate an m-sequence oflength 63 (L=6), one can use a primitive polynomial of degree 6 such asx⁶+x+1. This gives the three connections to the modulo-2 adder 320 inFIG. 3.

The generated length-63 sequence according to the shift register of FIG.3 is m₆₃=[1 0 0 0 0 0 1 0 0 0 0 1 1 0 0 0 1 0 1 0 0 1 1 1 1 0 1 0 0 0 11 1 0 0 1 0 0 1 0 1 1 0 1 1 1 0 1 1 0 0 1 1 0 1 0 1 0 1 1 1 1 1]. The{0, 1} values in the m-sequence are then be converted to antipodalvalues {1, −1}. The periodic autocorrelation property of anantipodal-valued m-sequence of length N is illustrated in FIG. 4. It canbe seen that ρ(0)=N, and ρ(n)=−1 for −N+1≦n≦−1 and 1≦n≦N−1. In general,the periodic autocorrelation of a sequence of length N is defined as:

${{\rho(n)} = {\sum\limits_{i = 0}^{N - 1}{{m(i)} \oplus {m\left( {\left( {i + n} \right)\%\mspace{14mu} N} \right)}}}},$where (a % b) represents the remainder of (a/b). Here, the correlationis calculated over the entire duration of the sequence.

The autocorrelation properties of the m-sequence make it attractive as apilot sequence for time acquisition or channel estimation. However, inmultipath environments, aperiodic correlations may impact performancemore. As illustrated FIG. 5, the pilot sequence may be received via atwo-path channel. In order for the receiver to estimate the channelcoefficient of the first path, the receiver aligns its local copy of thepilot sequence according to the arrival time of the 1st path andperforms correlation. In this process, interference from the second pathis picked up via the aperiodic autocorrelation between the pilotsequence received via the 2nd path and the receiver local generatedsequence. Here “aperiodic autocorrelation” refers to a correlationresult when the correlation interval less than the full sequence length,as pictured in FIG. 5. Although the m-sequence has very good periodicautocorrelation properties, it is not designed to have particularly goodaperiodic autocorrelation properties.

To take full advantage of good sequence autocorrelation properties, agood pilot sequence can be based on repeating a basic sequence, e.g., anm-sequence or any other sequences of good autocorrelation properties.This is Illustrated in FIG. 6, where a pilot sequence is obtained byrepeating a basic sequence several times. At the receiver, the basicsequence can be used to pick up the channel coefficient of path 1 (asshown). In this case, the interference from path 2 will also be pickedup; however in this case through a periodic autocorrelation. As notedabove, if an m-sequence is used as the basic sequence, such periodicautocorrelation will have values −1. To achieve this effect, the basicsequence can be repeated an integer number of times, as shown in FIG. 6,or a fraction of the basic sequence can be repeated.

As discussed above, m-sequences can serve as the basis for a good TDMpilot symbol sequence. However, as noted earlier, it is also desirablethat a TDM pilot signal maintains orthogonality with the CPICH. Assuggested above, this may be accomplished by spreading the TDM pilotsymbol sequence using an OVSF code, orthogonal to that used by CPICH,and further scrambling the spread sequence by a WCDMA long code. Onesuch sequence generation process is illustrated in FIG. 7, whichillustrates functional components of one or more processing circuits700, such as might be found in base station nodes 120 of FIG. 1. Thoseskilled in the art will appreciate that processing circuits 700 maycomprise, for example, one or more microprocessors, microcontrollers,digital signal processors, and/or customized digital hardware, as wellas one or more memory circuits configured to store program instructionsfor execution by corresponding processing elements, program data,configuration, and the like. Whether implemented in pure hardware or acombination of hardware, processor elements, and corresponding programinstructions, the processing circuits 700 include two spreaders 710,which spread a TDM pilot symbol sequence and a CPICH pilot symbolsequence, respectively, according to corresponding channelization codes.The two spread sequences are added with adder 720, and then scrambled atscrambler 730, using a common scrambling code. The resulting pilotchannel signal is then passed to radio-frequency (RF) transmittercircuitry 750 for transmission to one or more mobile stations, such asmobile station 110 in FIG. 1.

In more detail, the processing circuits 700 take a pre-determined symbolsequence p′_(k), spread the symbol sequence by an OVSF code having aspreading factor of two, and combine the spread TDM pilot symbolsequence with a spread CPICH pilot symbol sequence. The combinedsequences are then scrambled using a common scrambling code sequences_(k), which may be, for example, a WCDMA long code. In the processillustrated in FIG. 7, the TDM pilot symbol sequence is spread by thespreading factor 2 OVSF code [1, −1], which ensures orthogonality withCPICH. Thus, in some embodiments according to this approach, asymbol-level TDM pilot sequence of 128 bits is first spread by [1, −1],and the spread chips are further scrambled by the long code, whichvaries from slot to slot.

Several variants of the approach described above are possible. Forexample, a symbol-level sequence according to some embodiments isobtained by taking the modulo-2 sum of the long code and an extendedm-sequence. In these embodiments, the desired output sequence includesan extended m-sequence p₁₂₈ (128 chips long), which is obtained based ona basic sequence of length-63 m-sequence:p ₁₂₈ =[m′ ₆₃ ,m′ ₆₃ ,m′ ₆₃(0),m′ ₆₃(1)]where m′₆₃(i) is the ith bit of m′₆₃, which is 1−2m₆₃. Note that in thisformulation, the elements of p₁₂₈ and m′₆₃ takes on values {1, −1},i.e., a logical value 0 in m₆₃ is mapped to 1 in m′₆₃, and logical value1 in m₆₃ is mapped to −1 in m′₆₃. Given that s_(k) is the long(scrambling) code during the interval for the last 256 chips of slot k,then the input symbol-level sequence p′_(k) for generating the desiredoutput TDM pilot sequence p₁₂₈ in slot k is given byp_(k)′(i)=p₁₂₈(i)s_(k)*(2i), for i=0, 1, . . . , 127, where p_(k)′(i) isthe ith element of the symbol-level sequence p′_(k), and p₁₂₈(i) ands_(k)(i) are the i-th element of the extended m-sequence p₁₂₈ and longcode sequence s_(k), respectively. Here, it is assumed that the elementsof s_(k) have unity amplitude, such as values {1,−1} or unity-amplitudeQPSK values.

Those skilled in the art will observe that the extended m-sequence p₁₂₈that is desired to appear in the output is pre-scrambled by theconjugate of even (or odd) values of the scrambling sequence, so thatsubsequent scrambling will leave the desired extended m-sequence valuesfor the even (or odd) chip values. Thus, spreading and scrambling thesymbol-level sequence p′_(k) according to FIG. 7 results in the eventualTDM pilot sequence:z=(p ₁₂₈(0),−p ₁₂₈(0)s _(k)*(0)s _(k)(1),p ₁₂₈(1),−p ₁₂₈(1)s _(k)*(2)s_(k)(3), . . . ).That is:

${z(i)} = \left\{ \begin{matrix}{p_{128}(i)} & {{i = 0},2,4,\ldots\mspace{14mu},254} \\{x(i)} & {{i = 1},3,5,\ldots\mspace{14mu},255,}\end{matrix} \right.$where x(i)=p₁₂₈(└i/2┘)s_(k)*(└i/2┘)s_(k)(i) and └q┘ represents the floorfunction of q (i.e., the closest integer smaller than or equal to q).Thus, the even-numbered chips of the final output sequence z corresponddirectly to the extended m-sequence p₁₂₈. The odd-numbered chips can bethought of as pseudo-random, due to the pseudo-random properties of thescrambling code.

In another embodiment, also based on an extended m-sequence of 128chips, a symbol-level sequence p′_(k) for each slot is obtained bytaking the modulo-2 sum of a pre-determined extended m-sequence and thelong code segment corresponding to that slot. The extended m-sequence(128 chips long) in this embodiment is obtained based on a basicsequence of length-127 m-sequence p₁₂₈=[m′₁₂₇,1], where m′₁₂₇(i) is thei-th bit of m′₁₂₇, which is 1−2m₁₂₇. Again, the elements of p₁₂₈ andm′₁₂₇ take on values {1, −1}, i.e. a logical value 0 in m₁₂₇ is mappedto 1 in m′₁₂₇, and logical value 1 in m₁₂₇ is mapped to −1 in m′₁₂₇. Theopposite mapping may also be used.

Any m-sequence of length 127 may be used in the previous formulation.For example, a primitive polynomial of degree 7, such as x⁷+x³+1, may beused in the formulation of a shift register-based m-sequence generator.This polynomial gives the three connections from delay elements 810 tothe modulo-2 adder 820 shown in the shift register 800 of FIG. 8. Inthis case, the generated m-sequence is: m₁₂₇=[1 0 0 0 0 0 0 1 0 0 0 1 00 1 1 0 0 0 1 0 1 1 1 0 1 0 1 1 0 1 1 0 0 0 0 0 1 1 0 0 1 1 0 1 0 1 0 01 1 1 0 0 1 1 1 1 0 1 1 0 1 0 0 0 0 1 0 1 0 1 0 1 1 1 1 1 0 1 0 0 1 0 10 0 0 1 1 0 1 1 1 0 0 0 1 1 1 1 1 1 1 0 0 0 0 1 1 1 0 1 1 1 1 0 0 1 0 11 0 0 1 0 0].

The remaining steps of generating the TDM pilot sequence are the same asthe first embodiment discussed above. Thus, if s_(k) is the long(scrambling) code during the last 256 chips interval of slot k, then thesymbol-level sequence for generating a TDM pilot sequence in slot k isp′_(k)(i)=p₁₂₈(i)s_(k)*(2i), for i=0, 1, . . . , 127, where p′_(k)(i) isthe i-th element of the symbol-level sequence p′_(k), and p₁₂₈(i) ands_(k)(i) are the i-th elements of the extended m-sequence p₁₂₈ and longcode sequence s_(k), respectively.

Spreading and scrambling the symbol-level sequence p′_(k) according toFIG. 7 in this case results in the eventual TDM pilot sequence:z=(p ₁₂₈(0),−p ₁₂₈(0)s _(k)*(0)s _(k)(1),p ₁₂₈(1),−p ₁₂₈(1)s _(k)*(2)s_(k)(3), . . . ).That is:

${z(i)} = \left\{ \begin{matrix}{p_{128}(i)} & {{i = 0},2,4,{\ldots\mspace{14mu} 254}} \\{x(i)} & {{i = 1},3,5,\ldots,\mspace{14mu} 255,}\end{matrix} \right.$where x(i)=p₁₂₈(└i/2┘)s_(k)*(└i/2┘)s_(k)(i). Basically, theeven-numbered chips are taken from the extended m-sequence and theodd-numbered chips can be thought of as pseudo-random, due to thepseudo-random properties of the scrambling code.

In still other embodiments, the TDM pilot sequence can be based on abasic sequence having a length of 63, 64, 127, or 128. Still other basicsequences may be considered. These basic sequences can be extended tolength 128, as needed, by repeating chip values in a manner similar tothat described in embodiments 1 and 2. This gives a sequence p₁₂₈. Then,the same procedures described above can be used to obtain thesymbol-level sequence (length 128), and the eventual pilot sequence(length 256).

In various embodiments, the basic sequence or the complete pilotsequence can be pre-generated and stored in the memory of a base station(NodeB, in 3GPP terminology) or user terminal. In some embodiments, thepilot sequence can be pre-scrambled first, e.g., using the base stationscrambling code. The illustrated steps are repeated for each slot of oneor more frames of a broadcast/multicast signal, although the processingneed not be synchronous (e.g., the processing for two or more slots maybe performed ahead of time). In any case, the process for each slotbegins, as shown at block 910, with obtaining a pilot symbol sequencefor the slot, such that the pilot symbol sequence varies for each slotof a given frame. In some embodiments, as was discussed in detail above,the pilot symbol sequence may be generated as a function of a portion ofthe scrambling code corresponding to the particular slot—because thescrambling code segment corresponding to each slot of a given frame isdifferent, the pilot symbol sequences generated from those segments willgenerally differ as well.

As shown at block 920, the pilot symbol sequence is spread, using achannelization code. In some embodiments, as was discussed earlier, thechannelization code is an orthogonal variable spreading factor (OVSF)code, selected so that the transmitted TDM pilot signal is orthogonal tothe transmitted CPICH signal. In some embodiments, this OVSF code has aspreading factor of 2, although other spreading factors may be usedinstead. As shown at block 930, the spread pilot symbol sequence isscrambled, using a scrambling code, to form the TDM pilot channelsignal; the TDM pilot channel signal is then transmitted to one or moremobiles. As shown at block 940, the TDM pilot channel signal istime-division multiplexed with one or more traffic channel signalstransmitted during the corresponding slot. The TDM pilot channel is alsocode-division multiplexed with a second pilot channel signal (e.g., theCPICH) transmitted during all slots of each frame.

As noted above, the TDM pilot symbol sequence, which is ultimatelyspread and scrambled, may be pre-calculated and stored in a memory, insome embodiments. In other embodiments, it may be generated as needed.FIG. 10 illustrates a technique for obtaining a pilot symbol sequenceaccording to some embodiments of the invention. As shown at block 912, a128-symbol sequence that includes at least one instance of amaximal-length sequence is generated (e.g., using a shift registerimplementation of an m-sequence generator) or retrieved from memory. The128-symbol sequence is then “pre-scrambled” by combining the 128-symbolsequence with the conjugates of a series of values from the scramblingcode. This is done so that the corresponding values of 128-symbol valuesshow up in the pilot channel signal generated by the process of FIG. 9,i.e., in the sequence produced by the spreading and scrambling processespictured at blocks 920 and 930.

Simulations of the processes described above have been performed toevaluate the benefits of using the proposed sequences. In thesesimulations, main-lobe to average side-lobe power ratio (MSPR) was usedas a performance measure, and it was assumed that random chips valuesare present preceding and succeeding a 256-chip pilot sequence in thereceived signal. After feeding the simulated received signal into apilot-sequence matched filter, the sidelobes of the matched filteroutput were measured over a delay window indicative of maximum multipathdelay uncertainties. Compared to the use of a pseudo-random sequence,performance enhancements in excess of 1 dB were observed.

Those skilled in the art will appreciate that the techniques andapparatus described above provide means for a WCDMA based MBSFN systemto support an enhanced channel estimation scheme required to operate ina SFN broadcast radio channel, while also supporting a short duty cycle,so that mobile terminal receiver parts need only be operational duringdurations as short as possible. Of course, those skilled in the art willalso appreciate that the present invention may be carried out in otherways than those specifically set forth herein without departing fromessential characteristics of the invention. Thus, embodiments of thepresent invention include methods according to the techniquesillustrated and more generally described above, as well as wirelesstransceivers, such as might be used at a base station node, configuredto carry out one or more of these techniques. The present embodimentsare therefore to be considered in all respects as illustrative and notrestrictive, and all changes conning within the meaning and equivalencyrange of the appended claims are intended to be embraced therein.

What is claimed is:
 1. A method for transmitting a broadcast/multicastsignal in a mobile communications network from a base station operatingin a Single Frequency Network (SFN) that includes one or more other basestations transmitting the broadcast/multicast signal, the methodcomprising: obtaining a first pilot symbol sequence for each slot of oneor more frames of the broadcast/multicast signal, so that the firstpilot symbol sequence varies for each slot of a given frame independence on a scrambling code corresponding to the slot;channelization code spreading the first pilot symbol sequence for eachslot; scrambling the spread first pilot symbol sequence for each slot,using the scrambling code corresponding to the slot, to form a firstpilot channel; and transmitting the first pilot channel signals so thatthe first pilot channel signals are time-division multiplexed with oneor more traffic channel signals transmitted during each slot; andtransmitting, during all slots of the one or more frames, a second pilotchannel signal that is code division multiplexed with the first pilotchannel signal.
 2. The method of claim 1, wherein obtaining the firstpilot symbol sequence for each slot comprises generating the first pilotsymbol sequence for each slot as a function of a portion of thescrambling code corresponding to the slot.
 3. The method of claim 2,wherein generating the first pilot symbol sequence for each slotcomprises pre-scrambling a pre-determined symbol sequence by theconjugates of a series of values from the scrambling code, so that thecorresponding values of the pre-determined symbol sequence appear in thefirst pilot channel signal, after spreading and scrambling.
 4. Themethod of claim 3, wherein the pre-determined symbol sequence comprisesat least one instance of a maximum-length sequence.
 5. The method ofclaim 4, wherein the pre-determined symbol sequence comprises one of:two concatenated instances of a length-63 maximum-length sequence,extended with two symbols to form a 128-symbol sequence; or a length-127maximum-length sequence, extended with one symbol to form a 128-symbolsequence.
 6. The method of claim 1, wherein channelization codespreading the first pilot symbol sequence for each slot comprisesspreading the first pilot symbol sequence for each slot with anorthogonal variable spreading factor (OVSF) code, wherein the OVSF codeis selected so that the first pilot channel signal is orthogonal to thesecond pilot channel signal.
 7. The method of claim 6, wherein spreadingthe first pilot symbol sequence for each slot with the OVSF codecomprises spreading the first pilot symbol sequence for each slot withan OVSF code having a spreading factor of two.
 8. The method of claim 1wherein the scrambling code is common to the SFN.
 9. A base station nodeconfigured to transmit a broadcast/multicast signal in a wirelesscommunications system, as part of a Single Frequency Network (SFN) thatincludes one or more other base stations transmitting thebroadcast/multicast signal, wherein the base station node comprises: aradio-frequency transmitter circuit; and one or more processing circuitsconfigured to: obtain a first pilot symbol sequence for each slot of oneor more frames of a broadcast/multicast signal, so that the first pilotsymbol sequence varies for each slot of a given frame in dependence on ascrambling code corresponding to the slot; channelization code spreadthe first pilot symbol sequence for each slot; scramble the spread firstpilot symbol for each slot, using a scrambling code corresponding to theslot, to form a first pilot channel signal; and transmit the first pilotchannel signals, via the radio-frequency transmitter, so that the firstpilot channel signals are time-division multiplexed with one or moretraffic channel signals transmitted during each slot; and transmit,during all slots of the one or more frames, a second pilot channelsignal that is code division multiplexed with the first pilot channelsignal.
 10. The base station node of claim 9, wherein the one or moreprocessing circuits are configured to obtain the first pilot symbolsequence for each slot by generating the first pilot symbol sequence foreach slot as a function of a portion of the scrambling codecorresponding to the slot.
 11. The base station node of claim 10,wherein the one or more processing circuits are configured to generatethe first pilot symbol sequence for each slot by pre-scrambling apre-determined symbol sequence by the conjugates of a series of valuesfrom the scrambling code, so that the corresponding values of thepre-determined symbol sequence appear in the first pilot channel signal,after spreading and scrambling.
 12. The base station node of claim 11,wherein the pre-determined symbol sequence comprises at least oneinstance of a maximum-length sequence.
 13. The base station node ofclaim 12, wherein the pre-determined symbol sequence comprises one of:two concatenated instances of a length-63 maximum-length sequence,extended with two symbols to form a 128-symbol sequence; or a length-127maximum-length sequence, extended with one symbol to form a 128-symbolsequence.
 14. The base station node of claim 9, wherein the one or moreprocessing circuits are configured to spread the first pilot symbolsequence for each slot by spreading the first pilot symbol sequence foreach slot with an orthogonal variable spreading factor (OVSF) code,wherein the OVSF code is selected so that the first pilot channel signalis orthogonal to the second pilot channel signal.
 15. The base stationnode of claim 14, wherein the one or more processing circuits areconfigured to spread the first pilot symbol sequence for each slot withthe OVSF code by spreading the first pilot symbol sequence for each slotwith an OVSF code having a spreading factor of two.
 16. The base stationof claim 9 wherein the processing circuit is further configured to use ascrambling code common to the SFN to scramble the spread first pilotsymbol sequence.
 17. A single-frequency broadcast/multicastcommunications system, comprising two transmitter units in adjacentcells of a cellular communications network, wherein the transmitterunits are configured to transmit one or more identical traffic channelsusing identical channelization and scrambling codes and to transmit anidentical first pilot channel signal, wherein the transmitter units eachcomprise a radio-frequency transmitter circuit and one or moreprocessing circuits configured to: obtain a pilot symbol sequence foreach slot of one or more frames of a broadcast/multicast signal, so thatthe pilot symbol sequence varies for each slot of a given frame independence on the scrambling code corresponding to the slot;channelization code spread the pilot symbol sequence for each slot;scramble the spread pilot symbol sequence for each slot, using ascrambling code corresponding to the slot, to form the first pilotchannel signal; and transmit the first pilot channel signal, via theradio-frequency transmitter, so that the first pilot channel signal istime-division multiplexed with one or more traffic channel signalstransmitted during each slot; and transmit, during all slots of the oneor more frames, a second pilot channel signal that is code divisionmultiplexed with the first pilot channel signal.